Methods of this type are well-known per se. They are employed in order to make cover layers for laminates composed of boards and at least one layer of impregnated paper. This cover layer should have both a predefined decor and also advantageous physical and/or chemical properties, such as resistance to abrasion and/or resistance to scratching as well as resistance to water and chemicals. As a result, expensive melamine resins are generally used for the impregnation.
These coated and/or impregnated paper webs are well-known per se and are used, for example as furniture covering films or for floor laminates. This then requires meeting severe demands in terms of quality.
The known methods impregnate the decor paper with melamine resin and use hot air to dry it. When pressed together with the substrate, for example particle board under heat and pressure, the melamine resin has the effect of making the surface scratch-resistant and enabling the decor paper to be reliably bonded to the substrate. In order to reduce costs, substantially cheaper urea resin can be used for an initial impregnation. Both faces of the impregnated web must then be coated with melamine resin to achieve the desired surface properties of the laminate and to enable bonding to the board. This is labor-intensive, and more of the expensive melamine resin is used than is required for the surface quality.
If only one face of the web impregnated with pure urea resin is coated with melamine resin, it is impossible to bond it to the substrate since the urea resin is already too highly cross-linked due to the higher temperatures required to dry the melamine resin and thus does not have sufficient adhesive strength.
It is desirable when impregnating paper webs to largely replace the melamine resin with significantly cheaper urea resin.
WO 2008/134823 [US 2010/0159708] describes a method of producing impregnated paper in which urea resin is used for impregnation. Drying is effected by near-infrared radiation, thereby largely precluding cross-linking of the resin. When the film impregnated with the urea resin is pressed together with the substrate, the low level of cross-linking allows bonding to be reliably effected. One disadvantage of the method is that it does not provide the requisite operational reliability. It is furthermore costly in terms of the electrical power needed. In the case of colored decor types, certain areas of varying colors are dried differently due to variations in absorption and reflection, with the result that the overall web has a nonuniform level of moisture.
A further problem is the fact that the urea resin does not exhibit the required mechanical strength and thus undergoes excessively rapid wear. It is therefore necessary to provide one surface of the laminate with a qualitatively higher-grade layer composed, for example of melamine resin. This can be achieved in the case of floor laminates by using so-called overlay papers that are provided with special properties by special coatings and/or impregnations. These overlays are too costly for furniture since the quality of the surface must meet somewhat less stringent requirements. Coating the paper webs impregnated with urea resin subsequently with melamine resin is impossible since the high temperatures used for the prior cross-linking of the melamine resin have a negative effect on the urea resin which undergoes excessive cross-linking in the process and is thus no longer able to bond.